ci: Change host shard count to 12 - #5857
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Twenty concurrent jobs per host run is a large share of the runner pool, and the queue shows it: shards have been observed waiting up to 242s for a runner while the pool works through the backlog. Fewer, longer shards is the trade. Setup is ~195s per shard and largely irreducible — four experiments on CS-12583 established it is bound by the runner's cores rather than by latency, so parallelising or pre-computing it moves the cost rather than removing it. Each added shard therefore buys a shrinking slice of test time for a fixed ~3 minutes of machine time and one more slot held. Against the ~8,665s of test work in run 32523921998: 20 shards models to ~10.5 minutes and 3.49 machine-hours, 12 to ~15.3 minutes and 3.06. The wall-clock figure overstates the real cost, since it assumes every shard starts immediately, which is the assumption the queueing disproves. The count lives in three places that have to move together — the matrix list, `shardTotal`, and `--shard-count` on the timings generator, whose drift gate predicts the slowest shard. A comment now says so. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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Host Test Results 1 files 1 suites 1h 46m 49s ⏱️ Results for commit 72f6b2c. |
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Claude explanation
Twenty concurrent jobs per host run is a large share of the runner pool, and the queue shows it: shards have been observed waiting up to 242s for a runner while the pool works through the backlog.Fewer, longer shards is the trade. Setup is ~195s per shard and largely irreducible — four experiments on CS-12583 established it is bound by the runner's cores rather than by latency, so parallelising or pre-computing it moves the cost rather than removing it. Each added shard therefore buys a shrinking slice of test time for a fixed ~3 minutes of machine time and one more slot held.
Against the ~8,665s of test work in run 32523921998: 20 shards models to ~10.5 minutes and 3.49 machine-hours, 12 to ~15.3 minutes and 3.06. The wall-clock figure overstates the real cost, since it assumes every shard starts immediately, which is the assumption the queueing disproves.
The count lives in three places that have to move together — the matrix list,
shardTotal, and--shard-counton the timings generator, whose drift gate predicts the slowest shard. A comment now says so.